Methyl mercaptan is a well known intermediate for the production of organic compounds, such as sulfur containing amino acids, pesticides and dyes. Industrially, methyl mercaptan, also known as methanethiol, is produced mainly for the synthesis of methionine, a widely used feed supplement for poultry.
Methyl mercaptan is commercially produced by the heterogeneously catalyzed gas phase reaction of methanol and hydrogen sulfide. For example, EP-B-0832878 and DE-C-19654515 disclose a methanethiol preparation method based on the reaction of hydrogen sulfide (H2S) with methyl alcohol (CH3OH). EP-A-167,354 discloses a synthesis pathway based on the reaction of hydrogen sulfide with carbon monoxide (CO), wherein titanium dioxide (TiO2) was employed as carrier and nickel oxide (NiO) or molybdenum oxide (MoO3) as active component.
Chinese Patent Applications CN 1207957 and CN 1207958 disclose a series of catalysts useful for the methanethiol synthesis from high H2S-containing synthesis gas, wherein the active component (Mo—S—K-based species) comes from the precursor of K2MoS4 or (NH4)2MoS4 plus a potassium salt. In these Chinese patent applications, dimethylformamide [(CH3)2NCOH] and not water is chosen as solvent to dissolve the active component. The described process is hard to handle and expensive.
WO2005/040082A2 refers to a continuous process for the manufacture of methyl mercaptan using Mo—O—K based catalysts and a process for the preparation of a solid, preformed catalyst system. It is further described that the total selectivity of methyl mercaptan can be increased by at least 1% by lowering the total gas hourly space velocity.
EP-A-104507 describes a continuous process for reacting carbon oxides, sulfur or hydrogen sulfide, and hydrogen at elevated pressure and temperature. The reaction is carried out over a preformed, single-phase, solid catalyst system comprising a porous alumina containing support, on which a mixture of a manganese sulfide and at least one of an iron, nickel, zinc, chromium, cobalt, molybdenum or alkali metal sulfide is deposited. The described process is a continuous, vapor-phase reaction in the presence of a specified sulfur-containing or sulfide catalyst system containing manganese to produce methyl mercaptan with improved conversions and yields. It is stated that by using the described catalyst system, the methane formation is kept to a minimum, which should result in an improved economic process. Formation of inert by-products, such as methane, should be avoided because these inert materials are difficult to separate from the recycle gases. It would build up in the recycle gas streams and would have to be vented periodically.
Other by-products of the synthesis of methyl mercaptan from carbon oxides, sulfur or hydrogen sulfide and hydrogen include carbonyl sulfide, dimethyl sulfide, carbon bisulfide and dimethyl disulfide. Especially carbonyl sulfide formation should be kept to a minimum since carbonyl sulfide is an intermediate in the formation of methyl mercaptan. Low selectivities of carbonyl sulfide result in higher selectivities of methyl mercaptan thus improving the overall yield of methyl mercaptan and the whole economy of the process.
U.S. Pat. No. 4,665,242 describes a process for the production of methyl mercaptan by heating a gas comprising carbon monoxide and/or carbon dioxide, hydrogen sulfide and hydrogen in the presence of a catalyst based on a tungsten sulfide or rhenium oxide on an activated alumina substrate. In the process, unreacted gas is recycled to the feed gas stream, wherein the water which is formed during the reaction with the catalyst, is removed from the unreacted gas. The desiccation is carried out by passing the gas through a molecular sieve. Using a Re2O7/Al2O3 catalyst a maximum selectivity of 64.6% at a CO2-conversion of 28.0% is reported
Although innumerous attempts have been started to improve the selectivity and yield of methyl mercaptan manufactured from carbon oxides there is still a need for further improvements since high selectivities of methyl mercaptan at comparatively high conversions of carbon oxides are desired. Especially, carbon dioxide as Cl source for methyl mercaptan is attractive as the major by-product carbon monoxide can be easily converted into carbon dioxide thus increasing the overall selectivity for methyl mercaptan.